Flexible corrugated hoses are in common use with vacuum cleaners. Today, both canister and upright cleaners employ such hoses for interconnecting the vacuum source with a cleaning tool. Though vacuum cleaner hoses are of various configurations, the most common hoses are corrugated, defined by uniformly axially displaced ribs extending inwardly and outwardly of the hose. Oftentimes, the hoses are extendable or stretchable.
The inwardly extending ribs of the corrugated hoses are typically uniformly spaced and define uniformly spaced cavities or recesses therebetween. Additionally, the inner surfaces of the inwardly extending ribs are generally smooth. The uniform spacing, width, and depth or protrusion of the inwardly extending ribs have been known to generate audible sounds or whistles as air in the hose passes thereover when the hose is connected to a vacuum source. The audible sounds or whistles are set at a characteristic harmonic frequency which is a function of the width, spacing, and depth of protrusion of the inwardly extending ribs. As the hose is stretched or extended, the space, width, and protrusion depth of the inwardly extending ribs changes, but the uniformity of such features remains the same. Accordingly, the characteristic harmonic frequency of the hose changes with a resultant change in the tone of the whistle or audible sounds. Such whistling is highly objectionable to most consumers, since it is generally felt that noise incident to operation of a vacuum cleaner should be minimized.
It has been known that the characteristic whistling of vacuum cleaner hoses can be substantially eliminated by altering the space, width, and protrusion depth of the inwardly extending ribs and by configuring the inner rib surfaces to introduce turbulence and thereby eliminate the objectionable audible harmonics. However, the methodology for achieving such structure has been complex in nature and expensive to employ.
U.S. Pat. No. 2,600,254 presents methods for treating interior and exterior surfaces of extruded plastic tubing. A cylindrical die plug or a rotary die member is used to provide indentations on the interior wall surface of the tube. In either event, die members which are both interior and exterior of the of the tube are employed.
U.S. Pat. Nos. 3,015,857 and 3,185,182 provide means for forming a corrugated thermoplastic tube employing a vacuum drawing method. A vacuum forming element is positioned interior of the tube during the forming process. Apertures in the forming element communicate a vacuum to the interior of the tube. While it is possible that such apertures might tend to form small protrusions in the innermost diameter of the hose material, it is unlikely that such protrusions would be sufficient to preclude the generation of noise or whistling if the finished tube were used as an air conduit.
U.S. Pat. No. 3,240,645 teaches the formation of a corrugated hose employing a mandrel, wherein the mandrel has a plurality of small holes therein to allow for pressurization of the hose for ease of removal when the formation process is complete. While such holes might provide some type of imperfection on the interior surface of the hose, the same is not seen as being sufficient to preclude whistling or the like, nor provided for that purpose.
U.S. Pat. No. 3,605,817 presents a corrugated tube in which internal protrusions or "pimples" are formed by external die blocks. The protrusions or pimples "roughen" the interior surface of the tube to attenuate whistling and the like. The internal surfaces are formed by the provision of internal air pressure forcing the tube outwardly against an external die, without any additional mechanical forming process.
Finally, Canadian Patent 590,953 is of general interest in that it presents forming members having a series of small holes to contact a tube during the forming process. However, the holes are used to provide an escape path for water during a drying step in the forming process, and not to form any specific desired surface configuration.